Apex domains cannot have CNAME records.
Question and response
Question
dig intercom.com.sv. NS
Answer
intercom.com.sv. 5 NS ns2.intercom.com.sv.
intercom.com.sv. 5 NS ns3.intercom.com.sv.
intercom.com.sv. 5 NS ns1.intercom.com.sv.
Authority
The response has no authority records.
Additional
ns1.intercom.com.sv. 5 A 168.243.227.194
ns2.intercom.com.sv. 5 A 168.243.227.195
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0